1. Field of the Invention
This invention relates to a servo signal processing device provided in a recording or reproducing apparatus for an optical disc of a sample servo system, such as a magneto-optical disc, a compact disc or a write-once disc. More particularly, it relates to a focusing servo controlling device in which, when effecting sample servo by itself, such as during a focusing search, the output timing of the focusing servo signals is prompted for improving the phase allowance of the focusing servo loop for stabilizing focusing servo.
2. Description of the Related Art
The optical disc of the type having a sample servo system is well known. With the optical disc of this sample servo type, pre-set servo patterns are provided at a pre-set interval on the recording track. Recording data is recorded or reproduced between servo patterns as servo control is performed based upon playback signals of the servo patterns. The optical disc reproducing apparatus reproducing recorded data from an optical disc of the sample servo system is provided with a digital signal processor (DSP) for digitally processing servo error signals for effecting the servo control.
If reproduction of the recorded data is specified, the optical disc reproducing apparatus causes a laser beam to be illuminated on the optical disc via an optical system. Since the servo patterns are formed at a pre-set interval on the optical disc, the laser beam is irradiated on a servo area having the servo patterns recorded thereon and on a data area having recording data recorded thereon, in this order, as shown in FIG. 21a. One segment is made up of one servo area and one data area, such that the optical disc reproducing apparatus reproduces the recorded data with plural segments as one reproducing unit.
The reflected light is produced on irradiating the optical disc with the laser beam. The optical system receives this reflected light and accordingly forms RF signals while also forming tracking error signals and focusing error signals. These signals are sent to an A/D converter which is also fed with channel clocks generated by a phase locked loop (PLL) circuit based upon the repetition frequency of the servo patterns. Specifically, 216 channel clocks are formed for each segment. The A/D converter digitizes the RF signals, tracking signals and focusing signals based upon these channel clocks for forming and outputting RF data, tracking error data and focusing error data.
The RF data are supplied to a recording data detection system which then samples the RF data with the channel clocks of a pre-set frequency to reproduce recorded data and sends the reproduced data to e.g., an external equipment, such as a computer or a speaker. On the other hand, the tracking error data and the focusing error data are latched and outputted at the timing of reproduction of the servo area shown in FIG. 21a by a tracking error latch pulse and a focusing error latch pulse shown in FIGS. 21b and 21c, respectively.
The DSP is configured for operating based upon a software program routine, and captures latched tracking error data and focusing error data based upon sampling interrupt pulses supplied at the output timing of the latched error data, as shown in FIG. 21d. For correctly reproducing the recorded data, the tracking error data needs to be corrected more speedily than the focusing error signals. Consequently, the DSP processes the tracking error data, captured based upon the sampling interrupt pulses, before processing the focusing error data, similarly captured based upon the sampling interrupt pulses, as shown in FIG. 21e.
The DSP is fed with the channel clocks and counts the number of the channel clocks for each segment. The DSP outputs the tracking error data, processed as described above, at a timing when the count value reaches 128, or "80h" in hexadecimal notation, as shown in FIG. 21f. On the other hand, the DSP outputs the focusing error data, processed as described above, at a timing when the count value reaches 160, or "A0h" in hexadecimal notation, as shown in FIG. 21g.
The tracking error data and the focusing error data are supplied to respective servo systems. The servo systems effect phase compensation on the tracking error data and the focusing error data and accordingly servo-control the optical system in a direction of correcting the tracking errors and the focusing errors. This assures correct reproduction of recorded data in the "just-tracking" and in the "just-focusing" states at all times.
With the optical disc reproducing apparatus for reproducing the optical disc of the sample servo system, the tracking servo and the focusing servo are collectively controlled based upon the playback signals of the intermittently reproduced servo patterns. However, since it is necessary to improve response to tracking errors, tracking error data is processed and outputted in advance of the focusing error signals. If only the focusing servo is to be in operation, such as during focusing search or seek for effecting tentative focusing on the disc surface in advance of reproduction, the tracking servo is not required.
However, if the focusing error data is outputted in such a case at the same timing, it takes a lot of time from the time of sampling by the A/D converter until outputting of focusing error data from the DSP. The result is limitations imposed on the phase allowance of the servo loop and unstable servo states immediately after focusing servo engagement. Thus it may occur that a prolonged time elapses until the servo state becomes stabilized and, in the worst case, the focusing servo, once captured, is disengaged.
In view of this state of the prior art, it is an object of the present invention to provide a servo signal processing device in which, if the focusing servo is to be in operation by itself with the use of the optical disc of the sample servo system, the focusing error data is outputted more promptly for improving the phase allowance of the focusing servo loop for stabilizing the focusing servo.
Moreover, if the focusing servo is to be in operation by itself, e.g. during focus search or seek when tentative focusing is done on the optical disc surface prior to reproduction, the phase locked loop (PLL) circuit for the focusing servo becomes an open loop and is brought to the free running state. Thus, the frequency of the clocks for the focusing servo, outputted from the voltage controlled oscillator (VCO), is varied by about 20%. On the other hand, it takes from several msec to tens of msec from the time the focusing servo is engaged until the PLL circuit is locked. Consequently, if the focusing servo is captured under such conditions to close the PLL circuit to effect phase compensation of the focusing servo, arithmetic operations for the phase compensation filter become inaccurate such that accurate phase compensation becomes impossible. This problem is most acute directly after focusing servo capture when the focusing actuator amplitude is larger and hence the servo state becomes unstable and the focusing servo, once engaged, tends to be disengaged again even after phase compensation.
In view of the state of the prior art, it is another object of the present invention to provide a focusing servo controlling device in which, if the focusing servo is to be in operation by itself with the use of the optical disc of the sample servo system, phase compensation directly after focusing servo capture is accurate to enable stable focusing servo to be achieved.